Chapter 3: Initial Ugi Synthesis Attempts

3.1 Introduction

Several methods have been published on synthesizing diketopiperazines including the solid phase synthesis using resin bound peptides, which are cleaved off during cyclization (Tullberg 2006, Bianco 2000, Brunel 2004). Solution phase synthesis, however, provides a cheaper method. Exploitation of the Ugi reaction (4-UCR) provides a simpler method because the reaction is essentially carried out in a one pot fashion.

Our group’s initial attempts at preparing diketopiperazines began with the synthesis of DOPAL which was done by Khalid Mirza in our group (Mirza 2006B). This was a starting point after recognizing that many of the targets provided by Find-A-Drug contained this compound as the aldehyde component in the diketopiperazine (Bradley 2006A). The synthesis of DOPAL from adrenaline (Mirza 2006B) only yielded up to ten percent. With this, we began attempting the Ugi reaction using aldehydes that were commercially available and fairly inexpensive. Various aldehydes were used including phenylacetaldehyde and piperonal which are described in this chapter.

3.2 Experimental Section

3.2.1 Materials and Reagents
Phenylacetaldehyde, methanol, BOC-Gly-OH, piperonal, benzylisocyanide, and 5-methylfurfurylamine (5-MFA) were purchased from Sigma-Aldrich Chemical Company (Milwaukee, WI). Deuterated chloroform with 0.03% v/v TMS was purchased from Cambridge Isotope Laboratories, Inc. (Andover, MA). 2-Morpholinoethyl Isocyanide was purchased from Fluka Chemie (Steineheim, Switzerland).

3.2.2 Procedure Synthesis of the Ugi Product (20) Using Piperonal, 5-MFA, Benzylisocyanide, and Boc-Gly-OH (Holsey 2006A)

Scheme 3.1 Ugi Reaction Scheme for Piperonal, 5-MFA, Benzlisocyanide, and Boc-Gly-OH

To separate one dram vials was added 5-methylfurfurylamine (18) (111 uL, 1 mmol) piperonal (16) (150mg, 1 mmol), benzylisocyanide (19)(121 uL, 1 mmol), and Boc-Gly-OH (17) (175 mg, 1 mmol) and diluted to 1 mL with methanol. To a separate five dram vial each 1mL solution was added and mixed. The mixture was allowed to sit for 24 hrs. Evaporated solvent off using high vacuum. Added 2 ml deuterated chloroform (CDCl3) to dried product and transferred to an NMR tube using a Pasteur pipet. Synthesis of the Ugi Product (23) Using Phenylacetaldehyde, 5-MFA, 2-Morpholinoethyl Isocyanide, and Boc-Gly-OH (Holsey 2006B)

Scheme 3.2 Ugi Reaction Scheme for Phenylacetaldehyde, 5-MFA, Benzylisocyanide, and Boc-Gly-OH

To an NMR tube was added phenylacetaldehyde (21) (111 uL, 1 mmol) and 5-methylfurfurylamine (18) (111 uL, 1 mmol); the mixture was allowed to sit for 10 minutes. To a one dram vial was added Boc-Gly-OH (17) (0.175 g, 1 mmol), 2-morpholinoethyl isocyanide (22) (137 uL, 1 mmol) and CDCl3 (641 uL); this mixture was added to the NMR tube containing phenylacetaldehyde and 5-methylfurfurylamine. Allowed reaction to sit for 6 hrs (pending NMR availability) at room temperature. Took NMR spectra.

3.2.3 Instrumentation
All NMR spectra were taken on a 300 MHz Varian Inova at room temperature to determine the formation of the Ugi product. Proton NMR (HNMR) spectra were taken with 16 scans and a 3.74 second acquisition time for CDCl3. Carbon-13 (13C) NMR spectra (CNMR) were taken with 50 scans and a delay of 10 seconds.

3.3 Results and Discussion

3.3.1 Synthesis of the Ugi Product Using Piperonal, 5-MFA, Benzylisocyanide, and Boc-Gly-OH (Ugi synthesis attempt 1)

The region between 3.0-4.5 ppm in the proton NMR is expected to be "peak rich" because the majority of the starting materials contain methylene protons that are expected to come into resonance in this region (Figure 3.1):

· 5-MFA (singlet- methylene at 3.7ppm)
· Boc-Gly-OH (singlet- methylene at 4.0ppm)
· Benzylisocyanide (singlet- methylene at 4.6ppm)

Based on this it was difficult to determine whether or not the starting materials have been converted to the Ugi Product. This can also be said about the number of aromatic rings in all of the starting materials. The NMR spectrum of this reaction still shows a significant peak at 9.8 ppm, indicating that a significant amount of piperonal is still present (Figure 3.1). From this we determined that the Ugi product did not form in this experiment.


3.3.2 Synthesis of the Ugi Product Using Phenylacetaldehyde, 5-MFA, 2-Morpholinoethyl Isocyanide, and Boc-Gly-OH (Ugi synthesis attempt 2)
Phenylacetaldehyde turned out not be a good aldehyde for an effective Ugi synthesis. The aldehyde does begin to form an imine with 5-MFA, however it is accompanied by numerous side reactions that monopolize the reaction (Figure 3.2) (Mirza 2006C), (Bradley 2007A). This is evidenced by the appearance of new peaks in the HNMR spectra.

Figure 3.2 Evidence of side reactions in the HNMR spectra.

In addition, a reaction between Boc-Gly-OH and 2-morpholinoethyl isocyanide performed by Khalid Mirza in our group indicated that this particular isocyanide is not stable in the presence of Boc-Gly-OH (Mirza 2006A) (Bradley 2007B). During the synthesis of imidazolinium salts, Polyakov (Polykov 1983) suggests that the tertiary amine undergoes a cyclization in the presence of an acid.

3.4 Conclusion
Based on the NMR data, the Ugi products did not form in either attempt described here. Phenylacetaldehyde and 2-morpholinoethyl isocyanide are not good components for the Ugi reaction because of the side reactions that they undergo. DOPAL will likely behave similarly to phenylacetaldehyde in a Ugi reaction because both compunds are similar in structure. In addition, the aldehyde proton peak at 9.8 ppm indicates that some starting material remains in solution. It would be important to separate the reaction into its individual steps, starting with the formation of the imine. By separating the reaction, a better understanding of the reaction can be obtained including the kinetics of each step of the reaction.

3.5 Reference List

Bianco, A., Sonksen, C., Roepstorff, P, et al J. Org Chem. 65, 2179-2187 2000
DOI:10.1021/jo991818+ S0022-3263(99)01818-6

Bradley, JC 2006
Bradley, JC 2007A
Bradley, JC 2007B
Brunel, F. and Spatola, A “Synthesis of Diketopiperazines with on-resin N-methylation and cyclative release” J. Peptide Research Col 63 (3), 213, 2004 DOI: 10.1111/j.1399-3011.2004.00130.x

Find A Drug 2006
Holsey, A. 2006A
Holsey, A. 2006B
Kuo, M., et al. Targeting tuberculosis and malaria through inhibition of enoyl reductase” J. Biological Chem. Vol 278(23), 20851-9 2003
Mirza, K. 2006A
Mirza, K. 2006B
Mirza, K. 2006C

Polyakov, I. Synthesis of imidazolinium salts by cyclization of amino isonitriles” Chemistry of Heterocyclic Compounds 19(6), 684, 1983
Robbins, Jay H. Preparation and properties of p-hydroxyphenylacetaldehyde and 3-methoxy-4-hydroxyphenylacetaldehyde. Arch. of Biochem. and Biophys. 114 (3) 576-584, 1966
Tullberg, M. et al Microwave-assisted solid phase synthesis of 2, 5- diketopiperazines: solvent and resin dependence. J. Comb. Chem. 8, 915-22, 2006

Wikipedia 2006